This invention relates to a power brake unit for automotive vehicles including a brake valve which is adapted to meter pressure-transmitting fluid from a pressure source into a booster chamber to act upon a booster piston, and wherein a hydraulic transmission ratio reducing the actuating travel when the pressure source is intact is provided, the transmission ratio being without effect in the event of failure of the pressure source. A power brake unit of this type is known from German Pat. No. DE-OS No. 2,460,529.
In this known device, the hydraulic transmission ratio is rendered inoperative by being put out of circuit if the pressure source fails in order to thus alter the transmission ratio. This is necessary to permit the highest possible brake pressure to be built up with a relatively low pedal force and without the assistance of auxiliary energy, utilizing the maximum permissible pedal travel. If the transmission ratio was constant in a brake system designed for operation without auxiliary energy, the additional effect of the auxiliary energy would cause the pedal to be stepped down to the floorboard of the vehicle so that a further pressure build-up would be prevented as a result of this limited pedal travel.
The disconnectible transmission ratio is embodied in the power brake unit of the above-cited German Patent by arranging for the booster piston to have its one effective surface close to the master-cylinder piston in an intermediate pressure chamber, this surface being of a cross section greater than the effective surface of the master-cylinder piston facing it, and by arranging for the intermediate pressure chamber to be connectible with the working chamber in front of the master-cylinder piston through a valve arrangement.
A disadvantage of this known power brake unit is that the valve arrangement necessary for connecting and disconnecting the hydraulic transmission ratio necessitates substantial additional costs on the one hand and on the other hand it may be the cause of failure of the power brake unit by remaining in a permanently open or permanently closed position because of a defect. If such a defect occurs, sufficient deceleration of the vehicle is no longer possible using the pedal force alone when the auxiliary energy has failed, or, in the presence of the auxiliary energy, the brake pedal travels down to the floorboard, thereby limiting the further build-up of pressure.
Another disadvantage of the known power brake unit is that the hydraulic transmission ratio is situated between the master cylinder and the booster. In the normally intact power brake unit, this results initially in the translation of a high mechanical force with a small stroke into a corresponding lower force with a greater stroke. Consequently, the booster must be designed for forces which are not at all necessary. This results in an over-sized booster piston or an unnecessarily high level for the pressure source.